(1) Field of the Invention
The present invention relates to a process for converting a paper web, in which occurrence of curl is effectively prevented in a paper such as coated paper. More particularly, the present invention relates to a paper-converting process in which curling is prevented without formation of translucent specks or undulations or occurrence of the cockle phenomenon.
(2) Description of the Prior Art
In a paper mill, various coated papers such as printing papers, heat-sensitive recording papers and pressure-sensitive recording papers are prepared by applying a coating composition to a paper web in a coater. However, if a coated paper is curled, breaking of the web is readily caused, and therefore the manufacturing speed cannot be increased and the operating efficiency is reduced. Moreover, a product in which curling is caused is poor in the appearance characteristics and is difficult to feed or is likely to jam in the printing step or in a recording step.
The mechanism of occurrence of curling will now be described in brief. In general, one surface of paper is coated with a coating composition by a coater and the paper is then dried. Fibers and fiber clearances on the coated side of the paper contract more than those on the opposite surface when the coating composition is applied and subsequently dried. Therefore, the paper is curled with the coated surface being on the inner side.
Accordingly, a water applying apparatus for preventing occurrence of curl is disposed on a coater to apply an appropriate amount of water or an aqueous solution to the uncoated surface to prevent curling. However, complete prevention of occurrence of curling is impossible. Moreover, when the water applying apparatus is used, translucent specks appear as a new defect; water applied to the back surface of the coated paper arrives at the coated surface through pinholes and when the smoothing treatment is carried out by using a super-calender translucent specks are formed because the water is unevenly distributed in the paper and the areas of high moisture content in the paper web are crushed compared with the areas of low moisture content. In case of thin coated paper, there arises a cockle phenomenon; because of non-uniformity in the amount of water applied or in the water absorption, the water content in the paper becomes non-uniform and in the drying partial shrinkage or wavy deformation occurs. These defects, formation of translucent specks and occurrence of the cockle, result not only in degradation of the appearance of the paper but also in reduction of the recording quality, especially in case of heat-sensitive recording paper. For example, if translucent specks are formed, the specks are concave portions having a small thickness, and, therefore, these specks do not come in contact with a thermal head, and unrecorded spots of an image occur.
With this background, we carried out research with a view of eliminating incomplete correction of curl, formation of translucent specks and occurrence of the cockle phenomenon, especially research on the mechanisms of the generation of these undesirable phenomena in the case where a water applying apparatus is used. As a result we found the following facts. In a water applying apparatus customarily used in conjunction with a coating machine, for example, a two-roll type kiss coater, an applicator roll is rotated in a direction reverse to the direction of travel of the coated paper, and when water, conveyed in the form of a film on the outer surface of the applicator roll, is brought into a wedge-shaped water reservoir defined by the roll and paper, a hydrostatic pressure acts on the water in the water reservoir to intrude water unevenly into the paper layer from the back surface of the coated paper. It is considered that for this reason, the curl-correcting effect becomes insufficient and the cockle phenomenon or formation of translucent specks is caused.